A blower door test is a diagnostic procedure that measures the airtightness of a building. A calibrated fan is mounted in an exterior door frame and draws air out of the building, depressurizing it to 50 Pascals below outdoor pressure. Airflow required to maintain this pressure is measured in CFM (cubic feet per minute).
Results are reported as ACH50 (air changes per hour at 50 Pascals). Energy codes require new homes to meet maximum ACH50 targets — IECC 2021 requires 3 ACH50 or less in most climates. Very tight homes (passive house standard) achieve 0.6 ACH50 or less.
Blower door tests are used to: verify code compliance in new construction, diagnose air sealing opportunities in existing homes, and measure improvement after weatherization work. When combined with thermal imaging or smoke pencils, they locate specific air leaks for targeted sealing.